With a complete handshake the new attribute “bbio” is freed by calling the method tls_finish_handshake(),
which itself calls ssl_free_wbio_buffer(). When the handshake is not finished successfully, the “bbio” is not freed, and
therefore when calling SSL_free() the assert in ssl_free_wbio_buffer() fails.

The problem exists since OpenSSL 1.1.0 - there were no problems with OpenSSL 1.0.2x.